Breathing assistance apparatus

ABSTRACT

A humidifier and humidity sensor is disclosed for use with a breathing assistance apparatus. The humidity sensor preferably includes means to sense absolute humidity, relative humidity and/or temperature at both the patient end and humidifier end. The humidifier may also include provision to both control independently the humidity and temperature of the gases. Further, a chamber manifold is disclosed to facilitate easy connection of the humidifier to various outlets, inlets and sensors. A heated conduit is described which provides a more effective temperature profile along its length.

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/808,567 filed on Mar. 14, 2001 (pending) andentitled “Breathing Assistance Apparatus”.

BACKGROUND TO THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the use of an humidificationsystem particularly, but not solely, for providing respiratoryassistance to patients receiving mechanical ventilation or respiratorysupport.

[0004] 2. Summary of the Prior Art

[0005] A number of methods are known in the art for supplying humidifiedgases to a patient requiring breathing assistance. Such prior arthumidifiers generally comprise a source of pressurised air (or othermixture of gases), a humidification chamber including a source of waterand a heating means to vaporise the water, and a conduit to convey thehumidified gases to the patient or user.

[0006] For example U.S. Pat. No. 4,038,980 describes a “flashvaporisation” humidifier where water drips onto a low thermal massheater to create respiratory humidity. It mentions “control means may beprovided automatically to regulate the water supply rate in response tomeans sensing the relative humidity”, however they prefer a manualcontrol of water flow rate. Thus it incorporates a humidity sensor andcontrols the water rate, as opposed to controlling the amount ofelectrical heating.

[0007] U.S. Pat. No. 5,092,326 also describes the use of a humiditysensor in a humidifier. It describes a high frequency ventilation systemthat incorporates a heated humidifier and a humidity sensor, where theseare linked to a central microprocessor. Apparatus is disclosed tomoisten a gas mixture supplied to the airway, and a microprocessorcontrols the amount of moisture supplied to the gas mixture. While itdiscloses a humidity sensor at the patient airway, it doesn't describethe actual humidification configuration to be used.

[0008] U.S. Pat. No. 5,769,071 describes a humidifier incorporating aheat and moisture exchanger (HME), supply of water to the HME, heaterelement and humidity sensor. The humidity sensor can control humidityvia water supply rate or temperature (via the heater element). Also thehumidity sensor is described as being at the patient airway.

[0009] U.S. Pat. No. 5,988,164 describes a heated breathing tube systemfor use with a humidifier. This uses a relative humidity sensor (locatednear the patient) to control the amount of heating provided by theheated breathing circuit so that the gas is at a constant level ofrelative humidity. The heated breathing circuit may use eitherelectrical heating, or heating via warm recirculating water in a tube.Also described is a method of control of the electric heater wire orheated water tube based on the output of relative humidity sensor.

[0010] The previously mentioned U.S. Pat. Nos. 4,038,980 and 5,769,071both describe humidifiers where the humidification chamber is locatedclose (proximal) to the patient. These have the disadvantage ofintroducing weight, heat and complexity near the patient which isinconvenient and could be painful to the patient. Of the cited prior artonly U.S. Pat. No. 5,988,164 specifically describes the humidificationchamber as being located remotely from the patient.

[0011] There are several disadvantages of the prior art systems using ahumidification chamber located remotely from the patient. It is normallyassumed that gases leaving such prior art humidifiers are saturated withwater vapour (100% relative humidity). However there is no guaranteethat the gases leaving such humidifiers are in fact saturated with watervapour. In certain circumstances (e.g. with the incoming air alreadywarm), the gases leaving such humidifiers can be significantly less than100% relative humidity. This is because as they are typically controlledto achieve a desired outlet gas temperature, which in such cases may notbe much more than the incoming air.

[0012] Another drawback of the prior art systems is that condensationcan occur in the (sometimes heated) conduits connecting the patient tothe respiratory assistance equipment. This may occur if the temperatureprofile along such conduits is not even and allows some parts of theconduit to be colder than the gas at these points.

[0013] A third disadvantage of such prior art systems is where the gasleaving the humidifier is at 100% relative humidity it must be heatedimmediately by some form of conduit heater or it may lose heat throughthe walls of the conduit, which results in condensation and therefore adrop in the amount of absolute humidity contained in the gas.

[0014] Another fourth disadvantage of the prior art systems is the needfor a sensor very near to the patient, which adds to the weight and bulkof equipment at the patient's airway.

[0015] A fifth disadvantage of the prior art systems is thatintermittent or varying flow rates will cause the absolute humidity thatis generated by the humidifier to be uneven. This is because the flowrate is varying faster than any control loop that might operate in suchhumidifiers. Air which passes through the humidifier at a high flow ratehas had little time to be heated and humidified, while air that passesthrough the chamber at a low flow rate will be hotter and contain higherabsolute humidity. Consequently it is difficult for a conduit in suchprior art systems to transport these high humidity boluses withoutcondensation and consequent loss of absolute humidity.

SUMMARY OF THE INVENTION

[0016] It is therefore an object of the present invention to provide ahumidification system which goes some way to overcoming the abovementioned disadvantages, or which will at least provide the public witha useful choice.

[0017] Accordingly in a first aspect the present invention consists in ahumidification apparatus for humidifying gas for a patient or otherperson in need of such gas comprising:

[0018] an inlet receiving gas,

[0019] an outlet providing gas with a predetermined humidity and/ortemperature,

[0020] a humidifier configured to provide water vapour to said gaspassing through said humidification apparatus,

[0021] an air heater configured to directly heat said gas passingthrough said humidification apparatus in parallel to said humidifier,

[0022] at least one sensor configured to provide an indication of atleast two of, relative humidity, absolute humidity and temperature,

[0023] a controller or processor configured to energise said humidifierand said air heater to achieve a predetermined combination of any two ofabsolute humidity, relative humidity and temperature.

[0024] Wherein said sensor comprises an absolute humidity sensor forproviding an indication of the absolute humidity of said gases flow atleast one point in the flow path through said apparatus of said gasesflow, and said humidifier including a body of liquid water and said gas.

[0025] Wherein said humidifier comprises a metal spiral element to heatsaid body of water.

[0026] Wherein said humidifier comprises a heated porous ceramic memberadapted to be in contact with said body of water and said gas.

[0027] Wherein humidifier comprises a heated semipermeable membraneadapted to be in contact with said body of water and said gas.

[0028] Wherein said air heater having a humidification bypass, forallowing a portion of said gases to flow to pass from said inlet to saidoutlet substantially without humidification.

[0029] Wherein said humidification bypass includes a bypass conduit inat least partially passing through said body of water for conveying aportion of said gas from said inlet to said outlet, and a valve providedin said bypass conduit to thereby control of the portion of said gas insaid bypass conduit, the gas flowing through said bypass conduit beingheated by the surrounding said body of water.

[0030] Wherein said humidification bypass further having a bypassconduit for conveying a portion of said gas from said inlet to saidoutlet including a bypass heater adapted to heat the portion of said gasin said bypass conduit and/or said bypass conduit, and a valve providedin said bypass conduit to thereby control the portion of said gas insaid bypass conduit.

[0031] Wherein the restriction provided by said valve on the portion ofsaid gas in said bypass conduit is in use permanently set.

[0032] Wherein the restriction provided by said valve on the portion ofsaid gas in said bypass conduit is in use manually adjustable.

[0033] Wherein a humidification apparatus further comprises a flowsensor providing an indication of the instantaneous flow rate of whereinsaid control configured to control the restriction provided by saidvalve means on the flow rate of the portion of said gases flow in saidbypass conduit means based on said indication of instantaneous flow rateof said gases flow through said humidification chamber means, in orderthat the gases flow exiting from said humidification chamber means is ofsubstantially constant humidity.

[0034] Wherein said valve comprising an electromechanical actuatorconnected to a valve member wherein the energisation of saidelectromechanical actuator varies the position of said valve memberthereby varying the restriction provided by said valve means on the flowrate of the portion of said gas in said bypass conduit.

[0035] Wherein said valve comprising either a valve member connected toan elastic member or an elastic valve member wherein said valve beingpositioned in said gases flow at said inlet and the position of saidvalve member or said elastic valve member thereby determines the portionof said gas in said bypass conduit.

[0036] Wherein the position of said valve member or said elastic valvemember providing an indication of the rate of flow of said gas at saidinlet.

[0037] Wherein a humidification apparatus further comprises a conduit toconvey said gas from said outlet to a patient including insulationadapted to minimise the rate of heat energy lost by said gas in saidconduit, said controller adapted to energise said humidifier and saidair heater to minimise the condensation of the vapour from said gases insaid gases transportation pathway means while providing predeterminedlevels of absolute humidity.

[0038] In a second aspect a humidification apparatus for humidifying gasfor a patient or other person in need of such gas comprising:

[0039] an inlet receiving gas,

[0040] an outlet providing gas with a predetermined humidity and/ortemperature,

[0041] an air heater adjacent to said inlet for heating of said flow ofgas,

[0042] a humidifier configured to provide water vapour to said gaspassing from said heater to said outlet, in series with said heater,

[0043] at least one sensor configured to provide an indication of atleast two of, relative humidity, absolute humidity and temperature,

[0044] a controller or processor configured to energise said humidifierand said air heater to achieve a predetermined combination of any two ofabsolute humidity, relative humidity and temperature.

[0045] Wherein said air heater comprises a heater wire in a conduitconnected to said inlet.

[0046] To those skilled in the art to which the invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the scope of the invention as defined in the appended claims. Thedisclosures and the descriptions herein are purely illustrative and arenot intended to be in any sense limiting.

[0047] The invention consists in the foregoing and also envisagesconstructions of which the following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] One preferred form of the present invention will now be describedwith reference to the accompanying drawings in which;

[0049]FIG. 1 shows an example of an humidification system, comprised ofthree parts,

[0050]FIG. 2 shows a chamber which incorporates a metal element,

[0051]FIG. 3 shows a chamber using a porous material to provide aheating and humidifying function,

[0052]FIG. 4 shows a chamber using a semipermeable membrane,

[0053]FIG. 5 shows a chamber with a variable valve to adjust the ratioof gas which are bypassed,

[0054]FIG. 6 shows a chamber with an adjustable valve 30 where one partof the gas gets humidified while the other is heated,

[0055]FIG. 7 shows a chamber where the dry gas entering chamber ispre-heated,

[0056]FIG. 8 shows a chamber where the dry gas entering chamber isheated after leaving the chamber,

[0057]FIG. 9 shows a chamber combined with an unheated, well insulateddelivery tube,

[0058]FIG. 10 shows construction of a tube incorporating flexible PTCelements in a parallel wire configuration,

[0059]FIG. 11 shows a humidifier configuration using the tube in FIG.10,

[0060]FIG. 12 shows the chamber manifold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061]FIG. 1 illustrates a typical respiratory humidification system,comprised of three parts:

[0062] 1) a humidification chamber located at a distance from thepatient, which heats and substantially saturates gases flowing throughit;

[0063] 2) a delivery system consisting of a flexible tube which carrieshumidified gases from the humidification chamber 1 to the gas outlet 5;and

[0064] 3) a heater base which heats the humidification chamber 1 andprovides measurement and control functions.

[0065] The gas to be humidified flows into the chamber 1 from port 4 andleaves the delivery system 2 at gas exit port 5. Gas from exit port 5flows to a patient via a face mask or similar (not shown). The system iscontrolled using sensors located at positions 7 and 8—typicallytemperature probes. Dry gases at the gas input 4 are heated andhumidified by passing over the surface of hot water 6 in the chamber 1so that they are substantially saturated with water vapour when theyleave chamber 1 at exit port 10. Hot water 6 is heated by heater plate 9and the amount of heating is controlled so that the gas reaches apredetermined temperature at exit port 10. This temperature is measuredby sensor 7. Therefore the humidification chamber 1 acts to heat andhumidify the medical gases so that they are substantially saturated atthe output of chamber 1, and are at a predetermined temperature.

[0066] The gas delivery system 2 (also known as a delivery tube orbreathing circuit) consists of a flexible tube 11 containing a heater12, which may consist of a heated resistance wire. The gas from thehumidification chamber 1 passes through the tube 11 and is heated byheater 12 to offset heat losses through the walls of tube 11. The amountof heating applied to heater 12 is regulated so that the gas reaches apredetermined temperature at gas outlet 5, as measured by sensor 8. Thecontrol temperature at sensor 8 is usually higher than the controltemperature at sensor 7, so that the gas is heated along tube 11 toensure that condensation doesn't occur in the tube.

[0067] The system as described has gas entering gas inlet 4 from acontinuous flow gas source (not shown) and exiting the system throughgas outlet 5. However the system is equally applicable where the gassource is a ventilator, which creates intermittent flow patterns toprovide breaths to a patient. In this case gas outlet port 5 isconnected directly to gas inlet port 16. The patient is connected toport 17 via an endotracheal tube or similar (not shown). During patientinspiration dry gases from the ventilator enter the system at inlet port4, pass through chamber 1, delivery system 2, pass through wye-piece 13and reach the patient through port 17. During patient exhalation gasespass back through port 17, through wye-piece 13, tube 14 and leavethrough gas outlet port 18. Tube 14 may also be heated by heater 15 toprevent condensation.

[0068] Absolute Humidity Sensing

[0069] Humidifiers incorporating humidity sensors for display or controlhave been described in the prior art, however all used humidity sensorswhich were positioned at the patient airway. The current work describesnovel humidifier configurations incorporating a humidity generatingchamber located at a position which is remote from the patient, a heatedbreathing circuit to transfer humidity to the patient, and humiditysensors to control the level of absolute or relative humidity suppliedto the patient. These humidity sensors are to be located either:

[0070] 1) at the chamber outlet only,

[0071] 2) at both the chamber outlet and near the patient, or

[0072] 3) near the patient only.

[0073] One aspect of the present invention would be to use a humiditysensor as sensor 7. The purpose of humidity sensor 7 is to determine theabsolute amount of humidity which is being generated by chamber 1.Accordingly an absolute humidity sensor would be ideal for use as sensor7, although the use of a relative humidity sensor with associatedtemperature sensor could equally be used. This system has the advantageof creating a controlled level of absolute humidity at chamber outlet10, however this level of absolute humidity may not reach the patient ifcondensation is allowed to occur in tube 11.

[0074] An alternative system which would overcome this disadvantage isto use a second absolute humidity sensor at point 8 instead of atemperature sensor. The difference in absolute humidity between sensors7 and 8 allows the humidifier to determine whether condensation isoccurring between the two points. If the two absolute humidity sensors 7and 8 read the same level of absolute humidity then no condensation isoccurring in the tube. If the absolute humidity at sensor 7 is greaterthan at sensor 8, then the difference shows the rate of condensationthat is occurring.

[0075] One control strategy would be to control the amount of heatingprovided to heater 12 so that the absolute humidity difference isreduced to zero. However the tube may still contain mobile condensatebecause the humidity difference only describes the rate of condensation,not the absolute amount of condensate in the tube. Another controlstrategy is to remove this condensate and hence create a dry tube byheating heater 12 so that the rate of measured condensation is negative(i.e. condensation is being evaporated in tube 11) until the measuredcondensation rate reaches zero, indicating that all of the condensatehas been removed. The amount of heating can then be reduced until thesensors show that condensation has just started to occur, then theheating can be increased slightly to the optimum level. Drying out ofthe tube may be a continuous process, or may be initiated at regulartime intervals.

[0076] Another variation of the system shown in FIG. 1 would be to use atemperature sensor for sensor 7 and an absolute humidity sensor at point8. This system is simpler than having an absolute humidity at bothpoints 7 and 8. In operation the controller would have to adjust theamount of heating at heater 12 and heater plate 9 so that the correctlevel of absolute humidity was reached without condensate in deliverytube 12. In practice two separate control algorithms would be required,one to control the amount of heating occurring in tube 11 so that nocondensation occurred, and another to control heater plate 9 so that thedesired level of absolute humidity was generated in chamber 1. The twoalgorithms could work concurrently because the heater plate 9 willrespond slower than heater 12, so quick changes in absolute humiditywould indicate the action of heater 12. Sensor 7 provides a controlpoint for heater plate 9, but may not be needed.

[0077] Low Relative Humidity Chambers

[0078] All systems described so far have used a chamber 1 which attemptsto humidify the gas leaving gas outlet 10 to a high level of relativehumidity. While this condition isn't essential for the correct operationof the new humidification configurations just described because they usehumidity control, it was essential for the prior art humidifier wherecontrol is purely based on temperature. However there are someadvantages to be gained from using a chamber which heats gases to thecorrect absolute humidity, but at a low relative humidity (i.e. thetemperature of the gas is higher than the dewpoint of the gas, thereforethe gas is not saturated).

[0079] The first advantage is that it is easier to design a heateddelivery system to transport such a gas without condensation, since thegas doesn't need to be heated immediately after it enters the deliverytube to prevent condensation. Secondly, the use of low relative humiditygases leaving the chamber means that the heater element 12 can be ratedat a lower power than would otherwise be the case, as the gas alreadyhas a higher energy content and can tolerate a greater loss of energybefore the gas condenses in the tube 12. It may even be possible to usean unheated, well insulated breathing circuit instead of a heatedbreathing circuit if the chamber provides gas with enough energy. Notethat low relative humidity chambers can only be used if the heating tothe chamber is controlled using an absolute humidity sensor, not atemperature sensor, since otherwise the absolute humidity output wouldbe too low.

[0080] To this end, some humidification chamber configurations whichprovide a high temperature, low relative humidity gas output are shownin FIGS. 2-8. FIG. 2 shows a chamber which incorporates a metal element20 (e.g. a spiral scroll shape), but without wicking paper attached.This provides both dry heating (via the metal element) and heatedhumidification from the heated water 21. With this configuration thechamber 19 provides gas which is not saturated because some of theheating provided to the gas is dry heating via the metal scroll. Therelative humidity generated by the chamber is affected by the gas flowpath, scroll shape, dimensions, and the water level, and so is notreadily adjustable in use. However chamber 19 does give the condensatereducing advantages provided by a low relative humidity, controlledabsolute humidity output.

[0081]FIGS. 3 and 4 are alternative humidification chambers whichprovide low relative humidity, high temperature gases at their output.FIG. 3 shows a chamber using a porous material 22 (such as a porousceramic) containing water 23 to provide a heating and humidifyingfunction, while FIG. 4 shows a chamber using a semipermeable membrane 24to provide a barrier to the water 25 in the chamber. In both cases thesechambers provide dry heating via the porous or semipermeable material,as well as heated humidification from the water. In both cases the ratioof heating to humidifying is fixed and cannot be easily adjusted exceptby limiting the water supply.

[0082] FIGS. 5 to 8 show chambers that can supply gases at varyinglevels of relative humidity and temperature. In FIG. 5 a variable valve26 allows us to adjust the ratio of gas which passes through the drybypass tube 27 to that which flows across the surface of the water 28.The bypass tube passes under the water to heat the gas. The two gasstreams merge at the output 29. This is an example of a “parallel”system where the gas splits and takes two different paths to provideheating and humidification. In FIG. 6 the gas is again split into twogas paths using an adjustable valve 30. One part of the gas getshumidified by passing across the water 31 in chamber 32, while the otheris heated by heater 58, which surrounds tube 33. The gas paths merge atjunction 34.

[0083] The angle of variable valves 26 and 30 in FIGS. 5 and 6 may bepermanently set, may be manually adjustable, or may be automaticallyadjustable. One advantage of an automatically adjustable valve would beto provide a constant level of humidity out of the chamber when usedwith intermittent flow rates, for example when used with a ventilator.These flow patterns can be a problem because parts of the breath cyclecontain less humidity than other parts, due to the chamber providingless humidity at higher flow rates. One way to overcome this problem isto measure the instantaneous flow rate using a fast response flowsensor, and then rapidly adjusting the angle of the variable valve. Amore practical method of achieving this effect would be to spring-loadvalves 26 and 30 using springs 70 and 71. This would mean that low flowrates would mostly pass through the bypass tubes, while high flow rateswould operate the spring-loaded valve and allow more gas to pass acrossthe water in the humidification chamber. The angle of the spring-loadedvariable valve could also be used by the humidifier to measure the gasflow rate.

[0084]FIGS. 7 and 8 show alternative series configurations for lowrelative humidity chambers, where the dry gas entering chamber 35containing heated water 36 is either pre-heated via heater 37 in FIG. 7,or heated via heater 38 in FIG. 8 after leaving the chamber. In bothcases the heater provides dry heating to the gas and results in a lowrelative humidity, high temperature gas leaving outlet 39.

[0085] Any of the low relative humidity, high temperature chambers shownin FIGS. 2 to 8 can be used in conjunction with the humidity controlschemes described previously in this patent, but not successfully withthe prior art humidifier due to it being temperature controlled, nothumidity controlled.

[0086] Insulated Delivery Tube

[0087] Another facet of the invention is shown in FIG. 9. Here the lowrelative humidity, high temperature humidification system from FIG. 8has been combined with an unheated, well insulated delivery tube. Theincoming gas enters at port 35 into the standard humidification chamber36 containing water 37 which is heated by heater plate 38. The gas issubstantially saturated in the chamber then leaves the chamber throughgas outlet 39 and enters heated tube section 40 which heats the humidgas to a higher temperature, so that it has a low relative humidity. Thegas then passes through tube 41 which has an insulating layer 42 aroundit. Preferably the insulating layer is a thin jacket of stagnant airwhich reduces heat loss. As the high temperature gas, low relativehumidity gas passes through the insulating tube, a small amount of heatis lost through the tube walls, and therefore the gas cools. However theamount of heating applied to heater 40 is controlled, so that the gas isnever allowed to cool below its dewpoint, which would result incondensation within tube 41.

[0088] Several different sensor configurations are proposed. Firstly,sensor 43 could be an absolute humidity sensor which controls heaterplate 38 so that chamber 36 produces the desired level of humidity. Inone embodiment sensor 45 is a temperature sensor, which controls heater40 so that the gas passing sensor 45 remains at a certain desiredtemperature. If this temperature is greater than the dewpoint of the gasat sensor 43, then condensation should not occur in tube 41. Howeverthere may already be condensate in tube 41 when the humidifier is turnedon. If a humidity sensor is used for sensor 45 instead of a temperaturesensor, then the level of condensate occurring in the tube 41 can becontrolled. The algorithms described earlier in this patent fordual-humidity sensor control can be used with this system.

[0089] An alternative location for the absolute humidity sensor is atposition 44 instead of 43. The absolute humidity here should be the sameas at 43 because the gas has been heated and so hasn't lost anymoisture. However there may be advantages to placing the absolutehumidity sensor at 44, for instance due to better sensor operation in alow relative humidity environment. This location for the absolutehumidity sensor can be used with either a temperature or absolutehumidity sensor at location 45.

[0090] Humidifier Configurations without Any Patient Airway Sensors

[0091] Yet another aspect of this patent relates to removing the needfor a sensor at the patient airway. To remove this sensor safely, wemust be certain that the gas entering the delivery tube has a safe levelof temperature and absolute humidity, and that the surfaces inside thedelivery tube do not exceed safe temperature levels. This implies adelivery tube that has a constant internal wall temperature.

[0092] It would be desirable, therefore, to have a heated delivery tubewhich self-regulates its temperature at a desired level. The heatercould either be embedded in the wall of the delivery tube itself, or itcould lie inside the lumen of the delivery tube, or it could be wrappedaround the outside of the delivery tube. Such a heater could be madefrom positive temperature coefficient (PTC) material (such as“Winterguard” from Raychem Corp., Menlo Park, Calif. USA), so that theresistance of the heater increases if the heater is hot, resulting inreduced power. However the delivery tube may pass through more than oneenvironment, or may have localised drafts present on certain parts ofthe tube. If the PTC elements are arranged in parallel, then the fullbenefit of the PTC heater can be envisaged. If the PTC elements arearranged in parallel, then the cold portions of the tube will have alower resistance, which will result in more heat being dissipated. Thusthe tube will tend to regulate its own temperature.

[0093]FIG. 10 shows construction of a tube incorporating flexible PTCelements in a parallel wire configuration. The tube 48 is made of aflexible PTC material, which has two low resistive strip connections, 46and 47, on either side of it. This allows each portion of the tube toconsist of short conducting segments of tube connected in parallelbetween conductors 46 and 47. These segments are represented by dottedlines encircling the tube in FIG. 10. The conductors 46 and 47 areconnected to adjustable voltage source 49, which may be AC or DC. Thetube would have an outer layer (not shown) which provides electricalinsulation and thermal insulation to the tube. Each longitudinal segmentof the tube will be able to regulate its own temperature independentlyof the rest of the tube. To enhance this operation, it may be necessaryto provide parallel slots 50 running perpendicular to the axis of thetube, to eliminate electrical cross-connection between the different PTCsegments.

[0094] Although one specific PTC heated tube design has been envisagedand described, other PTC tube designs could be used. It may also be ofadvantage to create a PTC tube that has a differing temperature profilealong its length rather than a constant temperature profile. The PTCdesign could also be extended to incorporate PTC heaters in other partsof the patient breathing circuit, such as the flexible extension tubewhich is usually connected between the Y-piece (port 17 of FIG. 1) andthe patient's endotracheal tube. A further extension of the PTC tubeconcept would be into a self-heated and temperature controlledendotracheal tube.

[0095] The PTC tube described in FIG. 10 allows us to create ahumidifier which doesn't use any sensor at the patient airway. FIG. 11shows a humidifier configuration using this tube. Gas entershumidification chamber 52 via inlet port 51 and is humidified by water53, heated by heater plate 54. Absolute humidity sensor 55 controls theheater plate so that the gas passing sensor 55 is at a desired level ofabsolute humidity. PTC tube 56 is heated by an external voltage (notshown) so that the internal surface temperature is at a constant desiredtemperature, which is selected to be above the dewpoint of the gas. Thegas which leaves tube 56 at outlet 57 will therefore be near thetemperature of the tube, and containing the desired level of absolutehumidity which was controlled by absolute humidity sensor 55.

[0096] A variation of the system shown in FIG. 11 would be to use atemperature sensor at position 55. Another variation of a tube with aconstant internal wall temperature would a delivery tube with heatedwater or other fluid pumped through smaller conduits in the wall of thedelivery tube. Since the heated fluid has a high specific heat relativeto air, the temperature of the fluid remains fairly constant duringpassage through the delivery wall conduits.

[0097] Use of a Sensor/Heater Manifold

[0098] Traditional humidifiers have tended to use sensors that are probeshaped, so that they can be inserted through specifically designed holesin the side of the breathing circuit to measure temperature. However thehumidifier configurations that have been described in this patentincorporate many sensors around the chamber, so the use of a manifold 59as shown in FIG. 12 may be useful.

[0099] The humidification chamber 60 is a removable item which can beslid onto the humidifier base 61 as shown in FIG. 12. As the chamber 60is slid onto the humidifier base 61, its base makes contact with heaterplate 62 and its inlet and outlet ports 63 and 64 make contact withholes 67 and 68 inside the manifold 59. Dry air to be humidified entersthe manifold at port 65, passes out of the manifold through port 67, andflows through port 63 into the chamber 60, where it is humidified.

[0100] After leaving chamber 60 the humid gas passes through chamberport 64 into manifold port 68. Finally the humid gas leaves manifold 59through port 66 and passes to the breathing circuit.

[0101] The manifold may be a separate, removable assembly, or it may bean integral part of the humidifier base. It may contain temperaturesensors, humidity sensors, flow sensors, or a heater element. Thesewould be located inside the manifold 59 at positions 72 and 73. Themanifold 59 may be heated to prevent condensation of humid gas. It couldconnect to both chamber ports 63 and 64 as described, or it may onlyconnect to the outlet port 64. One advantage of using a manifold is thatmany sensors or heaters can be combined in a single, cleanable assembly,rather than requiring separate probes which need to be plugged into thebreathing circuit. This simplifies connection and setup for the user.Another advantage of a manifold is that the incoming dry gas temperatureand flow rate can easily be measured without additional probes andconnections.

[0102] Variations on the Described Configurations

[0103] Although absolute humidity sensors have been described with allof the different humidification schemes described in this patent,relative humidity sensors could also be used. This may involve slightlydifferent control algorithms to the ones described in this patent.Alternatively, a relative humidity sensor could be combined with atemperature sensor. This allows the absolute humidity to be calculatedfrom relative humidity and temperature, rather than being measureddirectly.

[0104] All of the novel humidification schemes that have been describedin this patent could be used with additional temperature sensors. Thesemay provide additional benefits such as providing a safety backup in theevent of a failed humidity sensor. Another benefit would be maintainingthe temperature being delivered to the patient within certain limits sothat the relative humidity is not too low, even though the absolutehumidity was acceptable.

[0105] Similarly it may be useful to measure the air flowrate throughthe humidifier, as this is an important parameter which affectshumidifier control. Therefore flow sensors could be incorporated withinany of the previously described systems. One useful prior art flowsensor construction would be to use a sensor based on heat loss from ahot element in the airstream. If a heated humidity sensor is used, theamount of heating that is required for the sensor to achieve temperaturecan be used to determine the gas flow rate.

[0106] Infection control is a prime consideration when designing medicalcomponents. To prevent bacterial colonisation of the components in thehumidification system, any parts which come in contact with the gasstream could be made out of antibacterial plastic. To preventcontamination of sensor probes, the probe ports could incorporate adisposable sheath which protects the probe from pathogens in thebreathing circuit. This would be particularly applicable to temperatureprobes. In general humidity probes need to have contact with the gasstream so a disposable sheath would be inapplicable to humidity sensors,unless they worked on optical principles, or unless the sheath was madeof water vapour permeable material, which did not allow the passage ofpathogens. The protective sheath could be an integral part of adisposable breathing circuit.

What we claim is:
 1. A humidification apparatus for humidifying gas fora patient or other person in need of such gas comprising: an inletreceiving gas, an outlet providing gas with a predetermined humidityand/or temperature, a humidifier configured to provide water vapour tosaid gas passing through said humidification apparatus, an air heaterconfigured to directly heat said gas passing through said humidificationapparatus in parallel to said humidifier, at least one sensor configuredto provide an indication of at least two of, relative humidity, absolutehumidity and temperature, a controller or processor configured toenergise said humidifier and said air heater to achieve a predeterminedcombination of any two of absolute humidity, relative humidity andtemperature.
 2. A humidification apparatus as claimed in claim 1 whereinsaid sensor comprising an absolute humidity sensor for providing anindication of the absolute humidity of said gases flow at least onepoint in the flow path through said apparatus of said gases flow, andsaid humidifier including a body of liquid water and said gas.
 3. Ahumidification apparatus as claimed in claim 2 wherein said humidifiercomprising a metal spiral element to heat said body of water.
 4. Ahumidification apparatus as claimed in claim 2 wherein said humidifiercomprising a heated porous ceramic member adapted to be in contact withsaid body of water and said gas.
 5. A humidification apparatus asclaimed in claim 2 wherein humidifier comprising a heated semipermeablemembrane adapted to be in contact with said body of water and said gas.6. A humidification apparatus as claimed in claim 1 wherein said airheater having a humidification bypass, for allowing a portion of saidgases to flow to pass from said inlet to said outlet substantiallywithout humidification.
 7. A humidification apparatus as claimed inclaim 6 wherein said humidification bypass including a bypass conduit inat least partially passing through said body of water for conveying aportion of said gas from said inlet to said outlet, and a valve providedin said bypass conduit to thereby control of the portion of said gas insaid bypass conduit, the gas flowing through said bypass conduit beingheated by the surrounding said body of water.
 8. A humidificationapparatus as claimed in claim 6 wherein said humidification bypassfurther having a bypass conduit for conveying a portion of said gas fromsaid inlet to said outlet including a bypass heater adapted to heat theportion of said gas in said bypass conduit and/or said bypass conduit,and a valve provided in said bypass conduit to thereby control theportion of said gas in said bypass conduit.
 9. A humidificationapparatus as claimed in claims 7 or 8 wherein the restriction providedby said valve on the portion of said gas in said bypass conduit is inuse permanently set.
 10. A humidification apparatus as claimed in claims7 or 8 wherein the restriction provided by said valve on the portion ofsaid gas in said bypass conduit is in use manually adjustable.
 11. Ahumidification apparatus as claimed in claims 7 or 8 further comprisinga flow sensor providing an indication of the instantaneous flow rate ofwherein said control configured to control the restriction provided bysaid valve means on the flow rate of the portion of said gases flow insaid bypass conduit means based on said indication of instantaneous flowrate of said gases flow through said humidification chamber means, inorder that the gases flow exiting from said humidification chamber meansis of substantially constant humidity.
 12. A humidification apparatus asclaimed in claims 7 or 8 wherein said valve comprising anelectromechanical actuator connected to a valve member wherein theenergisation of said electromechanical actuator varies the position ofsaid valve member thereby varying the restriction provided by said valvemeans on the flow rate of the portion of said gas in said bypassconduit.
 13. A humidification apparatus as claimed in claims 7 or 8wherein said valve comprising either a valve member connected to anelastic member or an elastic valve member wherein said valve beingpositioned in said gases flow at said inlet and the position of saidvalve member or said elastic valve member thereby determines the portionof said gas in said bypass conduit.
 14. A humidification apparatus asclaimed in claim 13 wherein the position of said valve member or saidelastic valve member providing an indication of the rate of flow of saidgas at said inlet.
 15. A humidification apparatus as claimed in claims 1or 2 further comprising a conduit to convey said gas from said outlet toa patient including insulation adapted to minimise the rate of heatenergy lost by said gas in said conduit, said controller adapted toenergise said humidifier and said air heater to minimise thecondensation of the vapour from said gases in said gases transportationpathway means while providing predetermined levels of absolute humidity.16. A humidification apparatus for humidifying gas for a patient orother person in need of such gas comprising: an inlet receiving gas, anoutlet providing gas with a predetermined humidity and/or temperature,an air heater adjacent to said inlet for heating of said flow of gas, ahumidifier configured to provide water vapour to said gas passing fromsaid heater to said outlet, in series with said heater, at least onesensor configured to provide an indication of at least two of, relativehumidity, absolute humidity and temperature, a controller or processorconfigured to energise said humidifier and said air heater to achieve apredetermined combination of any two of absolute humidity, relativehumidity and temperature.
 17. A humidification apparatus as claimed inclaim 16 wherein said air heater comprises a heater wire in a conduitconnected to said inlet.